Enamel



Patented June 13, 1 944 I @3515252' Alden J. Deyrup', westfleld, N. 5., assignor it a. du Pont de Nemours a Company, Wilmington,

Del, a corporatlcn ofzl lelaware v No Drawing; aprii i $1943 p 2. swarma claim iiii. ice-Ase;

[This invention relates to certainfimprovcd enamel compositions suitable for use in'the manufacture ofelectricai equipment. Moreparticularly, it relates to certain new and improved vitreous enamel compositions. having properties which render them-"particularlysuitablefor use; in the preparation of electrical resistors. and similar electrical apparatus. f

In themanufacture or electrical resistors it is usual to wind the resistance wire in coil form on a t'ubeoi ceramic base materiaL-suitableclips; clamps, ferrules, or other metallic faste'nings be-' ing provided at the :ends in" order to secure the resistance wire on the'c'eramic base, as well asto provide means for establishing electrical contact therewith. Over theentire surface of cera'mic base material andresistance wire there is then provided a layerof vitreous enamel which serves to hold the resistance wire firmly in place,

insulating the turns from each other and from outsidecontact. This enamel layer also serves dation.- I

I In the past in. reparing electrical resistors of this type, great difliculties have been encountered in manufacturing an enameled unit'which would satisfactorily withstand the severe stresses to which the members are subjected, bothduring the process in manufacture, and durlngsubse-- quent service conditions when'the resistor is in use. "In part, the severe stresses arise from the fact that electrical resistors are power dissipating units and; as commonly employed in most electrical circuits,'are requiredto dissipate a considerable amount of heat developed in the wire by I the. current of electricity passing therethrough This generation ,of-heat in the wire of the electricalre'sistor; which maybe gradual or, in some cases, very rapid, gives rise to mechanical stresses of the severest sort, these stresses resulting from the differences in thermal conductivities and coeflicients of expansion of the various component I parts comprising the resistor,

ceramic base, the resistor element proper, the -terminal bands or clips, and the vitreous enamel coating positioned thereon, all differ considerably in these two important thermal characteristics and, when heated, mechanical stresses of considerable magnitude invariably result.

The severe stresses are experienced not only during utilization of the electrical resistors, but

also during the manufacturing operations wherein it is necessary, in order to mature-the enamel thereon, to subject the assembly to successive firing and cooling operations. The heating incident to maturing the enamel, followed by the subsequent'cooling step, very often cause failureof the Thus the tubular I :its use as part ofthe electric insulating enameldestroying the usefulness I of the electrical resistor even before it is utilized in electricalapparatus. lziorecver, in someeiectricaleduipment such as in changes with great rapiderature due to external cously with the at-ion of heat deitseli incident to I circuit. Enamels which when applied anelsctricalresistor'of this type will successfully withstand thevarious stresses produced velopecl inthe resist ace wire stresses encountered during manufacture and use,

. I to protect the winding'against deteriorating in- I fiuences, such as those resulting from the pres I ence of .mois'ture, and electrolytes on the wire,- I as well as, those resultingfrom atmosphere oxi- -is designed to provide an adequate criterion of and which will not craze, Cracisspall off or break under these stresses have not hitherto been available commercially to American manunac turers otelectrical equipment. The high standarcls required in an enamel of this type will be appreciated when it is understood that even slight cracking, or any similar slight failure of the enamel coating, will expose the resistance wire underneath to electrolytic or other corrosive action, thus causing changes in resistance'or failure of the resistor within a relatively short period of time.

I One of the standard tests now utilized to determine the suitability of an enameled resistor element involves connecting the resistor in a suitable electric circuit and passing current therethrough under such circumstances that the external temperature of th vitreous coating is .raised to one in the neighborhood of 275 C.

The heating i then followed by rapid chilling by plunging the resistor into cold water. The test how the resistor will withstand severe service I conditons and, in practice, a satisfactory enameled resistor should wit stand shocks occasioned by a number of consecutive treatments of this kind without damage. Although many enameled wire-wound resistors are regularly manufactured which will withstand less severe tests and less exacting service conditions, at present there is no suitable enamel available for application to wire-wound resistors of all types which will withstand the stresses to which the member is subjected in'this test, or service conditions imposing stresses of comparable severity.

It is accordingly one of the objects of this invention to provide new 'and improved vitreous enamel compositions which may be applied to wire-wound resistors, such compositions not cracking, crazing, spalling off, or exhibiting failure of any type when the resulting enameled re- -sistcr is subjected to the foregoing severe test involving rep'eatedheating and quenching. Still another object of this invention is to provide insulating enamels which may be used for coating electrical resistor elements and similar electrical aircraft radio equip-' merit, the resistors be subjected to severe external temperature ity, these changes in influences occur equipment. which enamels may be readily applied thereto providing satisfactory insulation, the resulting enameled apparatus being characterized by a resistivity to thermal and other stresses of an order never previously attainable. These and still other objects of this invention will be apparent from the ensuing disclosure of certain preferred embodiments thereof.

I have found that enamel compositions containing large amounts of iron oxide, this oxide being fused into the vitreous enamel composition together with the oxides of lead, boron, aluminum and silicon, possess the superior resistance to crazing and to failure by thermal shock which permitstheir utilization for the specified purposes. Relatively large amounts of iron oxide are present in my improved vitreous enamels or glazes, amounts ranging from 9% to 20% by weight, these amounts being based on the total weight of the enamel composition.

My preferred enamels comprise the oxldic elements listed in the following table, these elements being present in the proportions specified therein. In all cases the percentages are by weight. based on the total weight of the enamel composition.

TABLE I a Per cent Iron oxide (F6203) 9 to 20 Lead oxide (PbO) 45 to 65 Boric oxide $2031---. 4 to 20 Alumina (A1203) 3 to 9 Silica (S102) 4 to 15 In addition to the five oxides noted, other ingredients may also be present such, for example, as one or more of the oxides of nickel, cobalt, and chromium. The latter metal oxides when incorporated in the enamel are beneficial in pro- Per cent Nickel oxide (N10) 1 to 5 Cobalt oxide (000).- 1 to 5 Chromic oxide (CrzOa) 1 to 5 Phosphorus pentoxide (P205) 1 to 4 These percentages are by weight, based on the total weight of the enamel.

In preparing enamels having compositions falling within the limits specified above various batches of raw materials may be melted until substantially homogeneous glassy melts are obtained. Table 11 lists five batches, batches A to E inclusive, which when melted yield the novel enamels having the calculated compositions given in Table III below. i

TABLE II Weight percentage batch composition A B C D E Red lead (Pbgol) 48. l 45. 7 48. 8 54.1 40. 5 Borlc acid (HsBOa) 18. 1 17:2 12. 2 6. S 30. 3 Ferric oxide (F9203) l5. 7 14. 8 12. 2 9. 5 12. l Kaolin (39.5% A110 46.5% 810:;

14.0 11 15. l 14. 3 15. 2 16. 8 12. 8 Flint (S101) 3.0 3.1 3.4 Cobalt oxide (C0304). 1.7 i 1.8 2.0 1 Nickel oxide (Ni:4O4) 1. 7 1.8 2.0 1. 5 Chromic oxide (CnOa) 1.7 1.8 2.0 1.5 Ammon. phosphate (NH4H2P04) 2.9 3.1 3.4

In these batches the various amounts of the different constituents are by weight.

The calculated compositions resulting from melting these batches are as follows, the same letter in each case serving to identify the same product in both tables.

TABLE III Calculated weight-percentages of enamel compositions Enamel A B o D E These percentages are by weight, based on the total weight of the enamel.

My improved vitreous enamels of the foregoing compositions are prepared in the usual manner by thoroughly mixing the constituents comprising the batch, and then melting the batch in any suitable furnace until a glass of uniform fluidity and sufiiciently low viscosity to permit pouring is secured. The melting temperature required to secure the homogeneous melt may vary somewhat but will normally fall within the range 1000 C. to 1300 C. Melting is continued long enough to insure uniformity of melt, and. it is especially important that the iron oxide constituent be melted into the glassy phase.

After the meltting iscomplete the melt is fritted in any suitable manner. Generally it is most convenient to pour the molten glass in a thin stream into water. The granular product thus resulting is then subjected to milling in a ball mill, in accordance with the well-understood procedure in the production of vitreous enamels, in order to reduce its particle size to the desired fineness.

In use, my enamel compositions are incorporated into a slip with water, there being added to this slip various other agents which, in accordance with the usual practice, may constitute dispersing and setting-up agents as well as temporary binders. Such additional agents may include, for example, bentonite, kaolin, ammonium acetate, water soluble gums,- ethylene glycol, etc., or any one or more of these agents.

The electrical resistor element or similar piece of electrical equipment, wound upon a suitable ceramic tube or base, to which the enamel is to be applied is then dipped into the slip, or the slip is applied thereto in any other convenient manner. A sumcient quantity of slip containing the enamel should adhere thereto to provide a satisfactory coating completely covering the resistance wire and supporting ceramic base. The electrical element and adhering slip is thendried and placed in a furnace and fired at a temperature in the range 550 C. to 750 C. A temperature within this range is maintained for a sufficient length of time to mature the enamel properly in place on the resistor. The resulting electrical resistor element, or similar piece of electrical equipment, coated with the adhering vitreous insulating enamel, will be found to meet the severe standard test previously described without crazing, cracking, spalling off or breaking. The ability of the enamel to withstand these severe conditions without even developing minute cracks is indeed remarkable, as no enamel previously available to the industry has exhibited similar ability.

In preparing the enamel compositions it is not necessary in all cases to incorporate either all y of the lead oxide or all of th silica ultimately milling and, in many cases, I have found it advantageous to add it this way as it permits application of the enamel to the resistance wire without excessive tearing or drawing up of the enamel during firing. This tearing or drawing up of the enamel results in a defect called swimming of the winding during firing, i. e., the displacement of the coils of the Wire through the action of the enamel. In addition to adding part of the lead oxide in this way as a mill addition it is also frequently desirable to add part of the silica during milling.

As an example of a typical enamel slip wherein part of the lead oxide is added subsequent to fusion and fritting as a mill addition the following is given. Instead of enamel Frit B in Table III any other enamel falling with the scope of my invention as previously set forth may b utilized.

Parts Enamel Frit B, Table III 93 Litharge (PbO) 5 Ethylene glycol 2 Water 50 This composition" containing the added lead oxide may be milled for three hours in a porcelain ball mill. The electric resistor is thoroughly covered, both metallic resistance wire and supporting base, with the slip, and upon fusion there results an adherent vitreous enamel of electrical insulating characteristics which will withstand the severe standard test employed in testing the commercial suitability of enameled resistors of this type.

I have tested numerous enameled resistor structures coated with. the enamel compositions previously described by heating them to 275 C. and then quenching them in saturated salt water at 0 C. Upon examination I have been unable to detect any evidence of failure. No cracking, spalling off, or crazing of the enamel has occurred. The availability of the enamel withstanding the severe test conditions described makes it possible to manufacture electrical apparatus of this type which may be used in communication equipment and for otherv purposes without any danger of failure of the electrical element or interruption of its service.

It is obvious that various changes may be made in my invention as above described which will nevertheless come within its scope. While the foregoing enamel compositions constitute examples of preferred enamels, it is obvious that variations in their compositions may be made without surrendering the outstanding advantages of the invention. Thus, the exact compounds used as the source of the oxidic constituents in the vitreous enamels may be varied, and suitable cheap and available sources may be selected. The melting procedure employed may be varied considerably provided homogeneity and thorough melting and fusion of the ferric oxide constituent is secured. It is accordingly my intention that the scope of my invention is to be construed in accordance with the appended claims.

I claim:

l. A vitreous enamel composition suitable for use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 45-65%; silica in amounts ranging from d-i5%; boric oxide in amounts ranging from 42Q%; alumina in amounts ranging from 3-9%; and ferric oxide in amounts ranging from 3-20%, said percentages being by weight based on the total weight of said vitreous enamel.

2. A vitreous enamel composition suitable for use in enameling electrical resistors which comprises: .lead oxide in amounts ranging from -65%; silica in amounts ranging from 1-15%;

alumina in amounts ranging from 3-9,%; boric oxide in amounts ranging from l-20%; ferric oxide in amounts ranging from 9-20%; nickel oxide in amounts ranging from 1- i%, said percentages being by weight based on the total weight of said vitreous enamel.

3. A vitreous enamel composition suitableior use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 45-65%; silica in amounts ranging from 1-15%; alumina in amounts ranging from 349%; boric oxide in amounts ranging from 4-20%; ferric oxide in amounts ranging from 9-20%; and cobalt oxide inamounts ranging from 1-5% said percentages being by weight based on the total weight of said vitreous enamel.

4 A vitreous enamel composition suitable for use in enameling electrical resistors which comprises: lead oxide in amounts ranging from 45-65%; silica in amounts ranging from 445%; alumina in amounts ranging from 3-9%; boric oxide in amounts ranging from i2il%; ferric oxide in amounts ranging from 59-20%; and an oxide of phosphorous in amounts ranging from 14%; said percentages being by weight based on the total weight of said vitreous enamel.

5. A vitreous enamel composition suitable for.

ALDEN J. DEYRUP. 

